The invention is relates to CVI/CVD furnace apparatus. More specifically, the invention is directed to methods and apparatus for inhibiting infiltration of reactive gas into insulation for a high temperature CVI/CVD furnace.
Processing of refractory composites takes place at elevated temperatures. Such processing includes CVI/CVD deposition of a binding matrix within a fibrous preform structure, and heat treating refractory composites. The reactant gas inlet that introduces the reactant gas into the furnace volume passes through the susceptor floor in the furnace. In prior art furnaces, the gas inlet is not sealed to the susceptor floor in order to allow it to move relative to the susceptor floor during heating and cooling of the furnace, and reactive gasses may leak from the volume enclosed by the susceptor and pass over and around furnace components outside the susceptor. Reactive gasses include the reactant gas used in the CVI/CVD process and gasses that are generated during heat treatment processes of refractory articles, for example carbonization of oxidized polyacrylonitrile fiber preforms. The reactive gas may attack furnace components. Porous insulation disposed around the susceptor is particularly susceptible to attack and degradation. Coated rigid insulation blocks have been used, but the coating tended to spall or crack over time and the coating became ineffective. Therefore, a method and apparatus is desired that will inhibit infiltration of reactant gas into the porous insulation over extended periods of time, preferably the life of the furnace.
According to an aspect of the invention, a method is provided for inhibiting infiltration of a reactive gas into porous refractory insulation during high temperature processing of refractory composites, comprising the step of:
sealing porous refractory insulation that may be exposed to a reactive gas during high temperature processing of refractory composites with a coating sufficiently impervious to said reactive gas to block penetration of said reactive gas into said porous refractory insulation, said coating comprising a first component that is absorbed into and bonds to said porous refractory insulation, and a second component overlying said first component that forms an impervious barrier to said reactive gas.
According to a further aspect of the invention, a method is provided for inhibiting infiltration of a reactive gas into porous refractory insulation during high temperature processing of refractory composites, comprising the step of:
sealing a susceptor lid to a susceptor within a furnace with a compliant gasket impervious to reactive gas during said CVI/CVD or heat treatment process.
Further aspects of the invention are presented in the detailed description and drawings that follow.